Here is how to see number of connected to the gateway users. Nothing special/interesting and I am sure somewhere in the SecureKnowledgeBase it is to be found but with recent licensing improvements people ask a lot about that.
VALS - real-time number of currently connected users.
PEAK - largest number of users seen since last reboot
fw tab -t userc_users -s
HOST NAME ID #VALS #PEAK #SLINKS localhost userc_users 73 1 3 0